The researchers say that men choose younger wives, and women choose older husbands is because they have evolved this way.
The researchers also state that the reason behind why men evolved this way is because of progeny, as younger wives maximise the chances of producing offspring.
The theory is based on the findings of a study that involved more than 11,600 Swedish men and women aged 45-55, and their partners.
They found that couples where the man was six years older than the woman had the most number of kids - an average of 2.2.
The researchers also noted that the reason why women fall for older men is because the latter represent resources and stability.
Study leader Martin Fieder said that the finding may help explain why men fall for younger women and vice versa.
"These findings may account for the phenomenon that men typically prefer and mate with women younger than themselves, whereas women usually desire and mate with men older than themselves," the Telegraph quoted him, as saying.
"We conclude that the age preference for the partner increases individual fitness of both men and women and may thus be an evolutionarily acquired trait," he added.
The researchers also noted that this trait is even visible in a second marriage where men and women both tend to choose younger partners.
Women however, choose men who are younger than their first partners yet slightly older than themselves.
"These findings support the reported age preferences of ageing men for increasingly younger women as well as of women for a partner just a little older than themselves," Mr Fieder said.
"We attribute the shift to a younger partner to a potential compensation for the fertility loss caused by the individuals' increasing age," he added.
The research is published online by the Royal Society journal Biology Letters.